Cardiac (ventricular) hypertrophy is an important adaptive physiological response to increased stress or demands for cardiac work. One of the early cellular changes that occurs after a stimulus for hypertrophy is the synthesis of mitochondria and expansion of myofibrillar mass (wall thickening) with a proportional increase in the size of individual cells, but no (or minimal) increase in the number of cells.
When the ventricle is stressed, the initial response is an increase in sarcomere length. This is followed by an increase in the total muscle mass. When the overload is severe; myocardial contractility becomes depressed. In its mildest form, this depression is manifested by a reduction in the velocity of shortening of unloaded myocardium or by a reduction in the rate of force development during isometric contraction. As myocardial contractility becomes further depressed, a more extensive reduction in the velocity of shortening of unloaded myocardium occurs, now accompanied by a decline in isometric force development and shortening. At this point, circulatory compensation may still be provided by cardiac dilation and an increase in muscle mass, which tend to maintain wall stress at normal levels. As contractility falls further, overt congestive heart failure, reflected in a depression of cardiac output and work and/or an elevation of ventricular end-diastolic volume and pressure at rest, supervenes.
The transition from hypertrophy to heart failure is characterized by several alterations in cellular organization. For example, normal hypertrophic cells have a large size with increased and well organized contractile units, as well as strong cell-cell adhesions. In contrast, pathologically hypertrophic cells, which also have large size and accumulation of proteins, display disorganization of contractile proteins (disarray of sarcomeric structures) and poor cell-cell adhesions (disarray of myofibers). Thus, in pathological hypertrophy, the increased size and accumulation of contractile proteins are associated with disorganized assembly of sarcomeric structures and a lack of robust cell-cell interactions.
Heart failure affects approximately five million Americans, and more than 550,000 new patients are diagnosed with the condition each year. Current drug therapy for heart failure is primarily directed to angiotensin-converting enzyme (ACE) inhibitors, which are vasodilators that cause blood vessels to expand, lowering blood pressure and reducing the heart's workload. While the percent reduction in mortality has been significant, the actual reduction in mortality with ACE inhibitors has averaged only 3%-4%, and there are several potential side effects.
ACE inhibitors have also been administered in combination with other drugs such as digitalis, which increases the force of the heart's contractions, and/or a diuretic, which helps relieve the heart's workload by causing the kidneys to remove more sodium and water from the bloodstream. However, at least one study demonstrated no difference in survival associated with the use of digitalis compared with placebo in patients with Class II-III heart failure. Additionally, diuretics can improve some symptoms of heart failure but it is not suitable as a sole treatment.
Additional limitations are associated with other options for preventing or treating heart failure. For example, heart transplantation is clearly more expensive and invasive than drug treatment, and it is further limited by the availability of donor hearts. Use of mechanical devices, such as biventricular pacemakers, are similarly invasive and expensive. Thus, there has been a need for new therapies given the deficiencies in current therapies.
One promising new therapy involves administration of neuregulin (hereinafter referred to as “NRG”) to a patient suffering from or at risk of developing heart failure. NRGs comprise a family of structurally related growth and differentiation factors that include NRG1, NRG2, NRG3 and NRG4 and isoforms thereof. For example, over 15 distinct isoforms of NRG1 have been identified and divided into two large groups, known as α- and β-types, on the basis of differences in the sequence of their essential epidermal growth factor (EGF)-like domains. NRG-1 is described, for example, in U.S. Pat. Nos. 5,530,109, 5,716,930, and 7,037,888; Lemke, Mol. Cell. Neurosci. 1996, 7:247-262; Peles and Yarden, 1993, BioEssays 15:815-824, 1993; Peles et al., 1992, Cell 69, 205-216; Wen et al., 1992, Cell 69, 559-572, 1992, Holmes et al., 1992, Science 256:1205-1210, Falls et al., 1993, Cell 72:801-815, Marchionni et al. 1993, Nature 362:312-8, the contents of which are incorporated by reference in their entireties. NRG-2 is described, for example, in Chang et al., 1997, Nature 387:509-512; Carraway et al., 1997, Nature 387:512-516; Higashiyama et al., 1997, J. Biochem. 122:675-680, Busfield et al., 1997, Mol. Cell. Biol. 17:4007-4014 and International Pat. Pub. No. WO 97/09425), the contents of which are incorporated by reference in their entireties. NRG-3 is described, for example, in Hijazi et al., 1998, Int. J. Oncol. 13:1061-1067, the contents of which are incorporated by reference in their entireties. NRG-4 is described, for example, in Harari et al., 1999 Oncogene. 18:2681-89, the contents of which are incorporated by reference in their entireties.
NRGs bind to the EGF receptor family, which comprises EGFR, ErbB2, ErbB3 and ErbB4, each of which plays an important role in multiple cellular functions, including cell growth, differentiation and survival. They are protein tyrosine kinase receptors, consisting of an extracellular ligand-binding domain, transmembrane domain and cytoplasmic tyrosine kinase domain. After NRG binds to the extracellular domain of ErbB3 or ErbB4, it induces a conformational change that leads to heterodimer formation between ErbB3, ErbB4 and ErbB2 or homodimer formation between ErbB4 itself, which results in phosphorylation of the receptors' C-terminal domain inside the cell membrane. The phosphorylated intracellular domain then binds additional signal proteins inside the cell, activating the corresponding downstream AKT or ERK signaling pathway, and inducing a series of cell reactions, such as stimulation or depression of cell proliferation, cell differentiation, cell apoptosis, cell migration or cell adhesion. Among these receptors, mainly ErbB2 and ErbB4 are expressed in the heart.
It has been shown that the EGF-like domains of NRG1, ranging in size from 50 to 64-amino acids, are sufficient to bind to and activate these receptors. Previous studies have shown that neuregulin-1β (NRG-1β) can bind directly to ErbB3 and ErbB4 with high affinity. The orphan receptor, ErbB2, can form heterodimer with ErbB3 or ErbB4 with higher affinity than ErbB3 or ErbB4 homodimers. Research in neural development has indicated that the formation of the sympathetic nervous system requires an intact NRG-1β, ErbB2 and ErbB3 signaling system. Targeted disruption of the NRG-1β or ErbB2 or ErbB4 led to embryonic lethality due to cardiac development defects. Recent studies also highlighted the roles of NRG-1β, ErbB2 and ErbB4 in the cardiovascular development as well as in the maintenance of adult normal heart function. NRG-1β has been shown to enhance sarcomere organization in adult cardiomyocytes. The short-term administration of a recombinant NRG-1β EGF-like domain significantly improves or protects against deterioration in myocardial performance in three distinct animal models of heart failure. More importantly, NRG-1β significantly prolongs survival of heart failure animals. These effects make NRG-1β promising as a broad spectrum therapeutic or lead compound for heart failure due to a variety of common diseases. However, there remains a need for more effective NRG peptides that can be used in a clinical setting for the prevention, treatment or delaying of heart failure and/or cardiac hypertrophy.